Field of the Invention
The present invention relates to an optical system and an image pickup apparatus including the optical system, which are suitable for, for example, a video camera, a digital still camera, a television (TV) camera, a monitoring camera, and a film camera using a silver-halide film.
Description of the Related Art
In general, in optical systems that are used in image pickup apparatus, an axial chromatic aberration and a lateral chromatic aberration are increased as the total lens length (distance from first lens surface to image plane) is reduced and the entire optical system is downsized. In telephoto optical systems, the chromatic aberration is increased as a focal length is increased. In order to correct the chromatic aberration over a visible wavelength range in such optical systems, it is required to correct chromatic aberrations of four wavelengths of a d-line, a g-line, a C-line, and an F-line of the Fraunhofer lines.
In general, as a method of reducing the chromatic aberration, there are known a method using an anomalous partial dispersion material as an optical material and a method using a diffractive optical element. However, a high-dispersion optical material has a higher partial dispersion ratio for the g-line and the d-line than that of a low-dispersion optical material. Therefore, when an achromatization method is employed to correct the chromatic aberration between the F-line and the C-line, the chromatic aberration of the g-line is apt to worsen.
Meanwhile, as a method of correcting the chromatic aberration of the g-line, there is well known a method using a diffractive optical element in an optical path. The diffractive optical element has an anomalous partial dispersion characteristic, and also has a small numerical value of 0.89 corresponding to the partial dispersion ratio for the g-line and the d-line. Thus, the diffractive optical element is effective for correction of the chromatic aberration of the g-line. Further, an absolute value of a numerical value corresponding to an Abbe number is a small value of 3.45. Thus, only minute optical power is caused by diffraction, and the chromatic aberration can be corrected with little influence on various aberrations such as a spherical aberration, a coma, and an astigmatism.
In view of this, there has been disclosed an optical system in which, because there is redundant power in a refractive optical element used together with the diffractive optical element, the total lens length is reduced and a glass material used for the refractive optical element is changed to a glass material having a relatively small specific gravity, to thereby reduce the lens weight (Japanese Patent Application Laid-Open No. 2010-145797).
Further, as another method of correcting the chromatic aberration in the visible wavelength range, there is known a method using in combination the diffractive optical element and the refractive optical element having an anomalous partial dispersion characteristic. There has been proposed an optical system configured to correct the chromatic aberration with use of this method (International Publication No. WO 2011/024258). In International Publication No. WO 2011/024258, there is disclosed a material range of an optical material having an anomalous partial dispersion characteristic, which is optimum for correction of chromatic aberrations of the four wavelengths of the d-line, the g-line, the C-line, and the F-line so that the chromatic aberration is corrected over the visible wavelength range in the optical system including the diffractive optical element.
In Japanese Patent Application Laid-Open No. 2010-145797, the total lens length of the telephoto lens is reduced by the chromatic aberration correction effect of the diffractive optical element, and various aberrations such as the spherical aberration worsened due to the increased refractive power of the refractive optical element in the telephoto lens are corrected by an aspheric lens. In this manner, various aberrations are corrected, and the entire optical system is reduced in size and weight.
When such an achromatization method as that of Japanese Patent Application Laid-Open No. 2010-145797 is used, the chromatic aberration can be corrected to a level without any practical problem in image pickup with use of the existing image pickup apparatus, but the method may be insufficient for the future image pickup apparatus compatible with high resolution and high image quality. In particular, it has been difficult to simultaneously correct the chromatic aberration of the g-line and the chromatic aberration between the F-line and the C-line. The reason is because the chromatic aberration caused in the diffractive optical element changes in proportion to the wavelength. The chromatic aberration caused in the diffractive optical element has a proportional relationship with the wavelength, and the slope of the chromatic aberration with respect to the wavelength between the F-line and the C-line is the same as the slope of the chromatic aberration with respect to the wavelength between the g-line and the F-line.
In contrast, the chromatic aberration caused in a general optical material changes in a curved manner with respect to the wavelength, and the slope of the curve tends to increase as the wavelength becomes shorter. Therefore, the slope of the chromatic aberration with respect to the wavelength between the F-line and the C-line differs from the slope of the chromatic aberration with respect to the wavelength between the g-line and the F-line. Thus, when the chromatic aberration caused in the general optical material is corrected by the diffractive optical element, and a power required for correcting the chromatic aberration between the g-line and the F-line is given to the diffractive optical element, the chromatic aberration between the F-line and the C-line remains. Conversely, when a power required for correcting the chromatic aberration between the F-line and the C-line is given to the diffractive optical element, the chromatic aberration between the g-line and the F-line remains.
Therefore, when the diffractive optical element is used in the optical system as in Japanese Patent Application Laid-Open No. 2010-145797, it has been difficult to simultaneously correct the chromatic aberration of the g-line and the chromatic aberration between the F-line and the C-line.
Meanwhile, in International Publication No. WO 2011/024258, in order to solve the problem in Japanese Patent Application Laid-Open No. 2010-145797, there is defined a material range of the optical material capable of simultaneously correcting the chromatic aberration of the g-line and the chromatic aberration between the F-line and the C-line in the optical system using the diffractive optical element. In this optical system using the diffractive optical element, the diffractive optical element and the refractive optical element made of an optical material having a desired material characteristic are used on at least one of an object side or an image side with respect to a stop.
However, the above-mentioned technology is based on the premise that the refractive optical element uses a resin material as the optical material having a desired material characteristic, and a refractive optical unit made of a resin material is arranged in the vicinity of an optical surface at which the diffractive optical element is arranged. Along therewith, as the refractive optical unit made of a resin material, a lens having a relatively large aperture diameter is used with its optical surface having a refractive power (with increased thickness of the resin material), and hence there have been problems in surface accuracy of the lens itself and in resistance to environment.